This invention relates generally to improvements in signal encoding/decoding methods and apparatus and, more particularly, to a new and improved digital encoding and decoding system for lower distortion, higher resolution, and increased dynamic range reproduction of analog signals while remaining compatible with industry standardized signal playback apparatus and standards not incorporating the decoding features of the present invention. In addition, recordings lacking the encoding process features of the invention are likewise compatible with playback decoders which do embody the invention, and are provided some enhancement.
Quite often a recording or communications system is standardized and its format cannot be readily altered without affecting a substantial quantity of equipment already in existence. Hence, adding information with supplemental codes may not always be practical unless provisions have been standardized for such insertions. Unfortunately, modern digital systems are not very expandable since data bandwidth, resolution, error correction, synchronization, ancillary data and other "housekeeping" information essentially occupy the entire digital capacity of the storage or transmission medium.
However, electronic equipment manufacturers and users of such devices continue to seek enhanced performance and more features from such standardized systems. An important example is the need to make a compatible recording well suited simultaneously for portable, automotive, television and audiophile markets. Today, many recordings are made for the most profitable market while other users suffer compromised sonics. The obvious conflicting performance requirements of different listening environments and the need for sonic improvement should desirably be implemented by a new system which is compatible with older systems and recordings.
Automobile and portable equipment are usually low cost and must operate in noisy environments. Hence, in such situations, a slightly restricted dynamic range playback is beneficial. Audiophile systems require utmost accuracy, dynamic range, and resolution beyond that which is available in the current standards. Thus, in any new compatible system, as provided by the present invention, encoded dynamics and slew rate modifications which achieve lowest distortion and best resolution for the audiophile when decoded, should also provide improved sonics for portable and automotive playback when not decoded.
Compact Disc pulse code modulation and other digital audio encoding schemes are good examples of highly developed and standardized systems which push signal conditioning and digital information limits. Most such digital systems originally evolved around then practical 2.5 to 3.5 mHz rotary head video recorder bandwidths. In such standards, the data bits with error correction and housekeeping entirely fill the available bandwidth. Accordingly, the need for a "smart" optimization technique, which does not rely upon increased bandwidth for its implementation, becomes apparent.
By way of background, let us consider a typical digital audio record-play system, its most frequently encountered components, operation, and difficulties. In its simplest form, the recorder includes a sampling switch and an analog to digital converter. The switch breaks the continuous analog signal into a series of voltage steps, each of which is converted to number groups or digital words. Digital level meters and simple communication systems often operate with just these functions in a single IC chip. Practical high performance record and playback systems require many added operations to prevent undesired internal and external analog-digital signal interactions, as well as beats and non-linear feedthrough between digital and analog frequencies. Well-known technologies to deal with these problems include sharp cut-off or "brick wall" low-pass filters, fast sample and hold circuits, and high common mode rejection amplifiers. Unfortunately, although these components and subsystems solve many problems, they also create others.
Briefly, in typical digital recording systems, low-pass filters ring, and if of analog construction, have pre-echo, are subject to sudden phase shifts near band edge, and have capacitors which often cause troublesome dielectric hysteresis effects. Sample and hold circuits have unpredictable timing and capture errors for different signal slew rates and also suffer from capacitor problems. Fast digital signals and the high speed amplifiers needed to handle them often create and are sensitive to ground currents which can cause audible strobe-beat effects. Digital reproducing systems have similar problems, along with spike or glitch generation caused by digital to analog conversion, and digital filter word length round off problems. Usually the recorder is designed to have state of the art performance while that of the reproducer degrades depending on the economies of "consumer" construction. These and other problems continue to plague modern high performance digital audio systems.
Unfortunately, such technical difficulties usually create jarring non-harmonic distortions, typically centered in the most sensitive and perceptive human hearing range. Often these distortions are caused by the highest, almost inaudible frequencies contained within the program material. Taking the ratio of high and low frequency hearing acuity into account, and the fact that sounds unrelated to the program material stand out, the presence of even an extraordinarily small amount of these distortions can be quite objectionable to the listener. Fortunately, often only very small corrections are needed to minimize some of these distortions. However, left as is, these distortion errors can combine to yield the equivalent of 13 to 14 bit performance accuracy from systems originally designed for 16 bit resolution. In practice, while some feel the advantages of current digital recordings outweigh the disadvantages of their distortion errors, many sophisticated listeners and audiophiles are not so tolerant.
Accordingly, those concerned with the development and use of digital signal encoding and decoding systems for analog signals have long recognized the need for a higher quality, lower distortion digital system for reproduction of such analog signals, which for all practical purposes is also compatible with existing equipment standards. The present invention fulfills all of these needs.